Refractometer Calibration

[jcook54]

For years I've used refractometer calibration fluid to calibrate my Vitalsine SR-6 refractometer. It's a great unit and I've had it for 14+ years. I was looking for some information about the Automatic Temperature Compensation (ATC) feature and went looking for the instructions. Turns out, the manufacture of this unit suggests calibration with just plan water and adjusting it to 0. You can read the calibration instructions through the link:

https://pentairaes.com/vital-sine.html

What gives? I was curious so I did a little experiment. I calibrated my Hanna Salinity checker (to 35 ppt), tested the refractometer vs. the salinity checker and got some interesting results. When I calibrated the refractometer with RODI water, the reading was 35 ppt. When I calibrated the refractomer with the 35 ppt calibration fluid, the reading I got was closer to 37 ppt.

When I calibrate the refractomer with plain water, per the instructions, the reading lined up with the freshly calibrated Hanna Salinity checker. When I calibrate the refractomer with the 35 ppt calibration solution, the reading was higher than the freshly calibrated Hanna Salinity checker. Should I be calibrating the refractometer with the RODI water moving forward? Have my tank been running closer to 37 ppt all these years? What do you think?

 

[york1]

I make my own calibration fluid. http://reefkeeping.com/issues/2004-06/rhf/index.htm
I have found most commercial calibration fluids are iffy at best. After having 2 Hanna salinity checkers I have come to the conclusion they are garbage and should not be the only way of checking salinity. Now i stick to my vee gee refractometer and Tropiv marin hydrometer to double check.

 

[ichthyoid]

I would follow the manufacturer’s instructions.

 

[Wmb0003]

I had a similar issue with a refractometer and a calibration solution that wasn't correct(probably my fault though.) I bought one of these(https://www.bulkreefsupply.com/high-precision-hydrometer-tropic-marin.html) and love the ease of readings, the difference on this unit between 1.025 and 1.026 is like an inch(just guessing here, but it is very easy to tell where you are at)

 

[lmm1967]

these instruments don't work on straight line scales like you would expect.

Calibrate per instructions - if you calibrate at 35ppm when they instruct to calibrate to 0 - you will be off.

Also - plain water means RODI or distilled - don't use tap water (there is salt and plenty of other dissolved stuff that conducts electricity in our water)

 

[Adam]

Conversely to all the recommendations, I think the reading is right when calibrating with 35ppt solution and reading low when using RO water.
I've read all over R2R that the Hannah checker reads 0.02 low consistently and mine does as well. If the Hannah reads 1.026 and the refractometer is calibrated with 35ppt and reads 37 that would also be inline with those findings.

I mix my water using the checker at 1.023. Before using, I double check with the refract before use and it's usually very close to 1.025/33ish ppt.

 

[ichthyoid]

Semantics,
Checking a device for deviation, at a single point of it’s variable range function, is a performance verification, not a calibration.

 

[lmm1967]

Conversely to all the recommendations, I think the reading is right when calibrating with 35ppt solution and reading low when using RO water.
I've read all over R2R that the Hannah checker reads 0.02 low consistently and mine does as well. If the Hannah reads 1.026 and the refractometer is calibrated with 35ppt and reads 37 that would also be inline with those findings.

I mix my water using the checker at 1.023. Before using, I double check with the refract before use and it's usually very close to 1.025/33ish ppt.

You put thought in to what you are doing

Many times, people (MYSELF INCLUDED - so please don't get mad) - don't think

 

[Dmac]

Have a question on this subject. I have the veegee stx3 and I noticed when I put rodi water to calibrate, the line doesn't move no matter how much I turn the screw. I assumed it was broken but when I double checked the reading against my hydrometer, it's correct. What gives?

 

[Adam]

Have a question on this subject. I have the veegee stx3 and I noticed when I put rodi water to calibrate, the line doesn't move no matter how much I turn the screw. I assumed it was broken but when I double checked the reading against my hydrometer, it's correct. What gives?

With saltwater, does turning the adjustment screw change the reading?

 

[york1]

The screw is just to lock down the adjustment collar not to change anything

 

[Dmac]

The screw is just to lock down the adjustment collar not to change anything

So what do you calibrate it with?

 

[Wmb0003]

loosening the screw will allow you to twist the piece that adjusts the line and then twist the piece that my fingers are on

 

[york1]

Yup you loosen the little set screw then turn the knurled ring to calibrate. Be careful not to over tighten the screw.

 

[Dmac]

Smh. Don't know why I couldn't find that out. Thanks guys